Who are my three friends?

I'm just a regular guy, but I have three best friends.

When I meet the first, he usually sends me two letters.

When I meet the second, she tends to leave me just as I am, sometimes having passed me a letter.

When I meet the third, he presents me with his final testament.

Who am I, and who are my friends?

Inspired by this riddle, but the solution is (almost certainly) different.

I believe that demonofthemist is very close.  Building on and branching out from his answer, you are:

A verb in English.  At the risk of getting too technical, you are the infinitive form of a regular verb in English.

Your three friends are:

The three tenses of verbs (past, present, and future).

When I meet the first, he usually sends me two letters.

Past Tense

The past tense of a regular verb in English is formed by appending “ed”.

(Note that I mentioned “regular” verb.  There are “irregular” verbs that do not follow this rule.  For example, the two verbs in the above sentence from the OP are both irregular:
“meet” → “met”,  “send” → “sent”)

When I meet the second, she tends to leave me just as I am, sometimes having passed me a letter.

Present Tense

The present tense of a verb in English is usually the base/root/infinitive, but an “s” gets appended when it is used with third-person singular subject.

For example: I type,  you type,  he types

A few irregular verbs don’t follow this pattern; for example,
I am,  you are,  he is;    I have,  you have,  he has

When I meet the third, he presents me with his final testament.

Future Tense

The future tense of a verb in English is (almost?) always formed by adding the auxiliary word “will”.

For example: I will type,  you will type,  he will type

The legal document specifying how a person’s estate should be distributed after he dies is called a “last will and testament” (emphasis added).

Alternatively, it could be that you are

Ebenezer Scrooge

and your “friends” are

the ghosts of Christmas past, present, and future.       :-)

• i'd go for scrooge :) – Sikorski Dec 9 '16 at 9:32
• Bingo, you got it! And you don't have to worry about irregular verbs, since the question says "I'm a regular guy :-) There are also a couple of other funny bits of wordplay - see if you can spot them. – Rand al'Thor Dec 9 '16 at 12:36
• I feel my answer as foolish now :D +1 – Techidiot Dec 9 '16 at 14:00
• @randal'thor:  No, you're too clever / tricky / cryptic / arcane for me — I can't spot any other funny bits of wordplay. – Peregrine Rook Dec 10 '16 at 6:10
• P.S. Pun not planned, but deliberately left in when detected.    :-)   ⁠ – Peregrine Rook Jan 17 '17 at 19:04

You are:

A Verb in English

Your three friends are:

Three forms of verb (Simple, Past & Past participle)

When I meet the first, he usually sends me two letters.

Past Form

When a verb is in past form it is usually appended a ED
Example : Gift -> Gifted, Walk -> Walked

But this is not mandatory
Example : Do -> Did, Go -> Went

When I meet the second, she tends to leave me just as I am, sometimes having passed me a letter.

Simple Form

When a verb is in simple or present form it is used as is, But get appended a S when used with third-person singular noun or pronoun.

Example : When he gets angry, his face turns red

Note : I'll mention that sometime ES is used instead on S, e.g. Do -> Does

When I meet the third, he presents me with his final testament.

Past Participle Form

The past Past participle form of a verb indicates past or completed action or time.

Example : Do -> Done, Go -> Gone, Awake -> Awoken

• Not bad at all. Maybe you got it. – stack reader Dec 9 '16 at 8:11
• Good one.... +1 – Techidiot Dec 9 '16 at 8:13
• Go --> Gone, I think... – Mithrandir Dec 9 '16 at 9:43
• Very very close, but the third part isn't quite there (doesn't fit with "testament"). Definitely +1, though I'll probably have to accept Peregrine Rook's answer instead. – Rand al'Thor Dec 9 '16 at 12:35
• Thanks guys! @randal'thor Sure! I overlooked "testament" and interpreted it differantly :) – demonofthemist Dec 9 '16 at 13:02

You might be:

Oxygen

And your 3 best friends are:

Hydrogen, Carbon, and Nitrogen (?)

When I meet the first, he usually sends me two letters

Hydrogen reacts with Oxygen to give H2O, so that's 2 Hs

When I meet the second, she tends to leave me just as I am, sometimes having passed me a letter

Carbon and oxygen do not usually react at normal conditions, but they do you get CO2, so that's only 1 C

When I meet the third, he presents me with his final testament

All I can think of is NOx, but that means "night" so I might be on the wrong track.

And the simple guy is

Octopus

When I meet the first, he usually sends me two letters.

Meet O(which are you) and it gives two letters CT

When I meet the second, she tends to leave me just as I am, sometimes having passed me a letter.

Meet C it gives two letters T and O(again)

When I meet the third, he presents me with his final testament

Meet T and it presents you with OPUS which is a synonym for testament or will.

And the tree friends are

OCT(October) :)

• Ironic it's December now, though, right? – EKons Dec 9 '16 at 18:17